It is well known to place balloon expandable stents into various vessels of a human subject. At the present time, a variety of stents are being used to act as a scaffold to prop open a stenosis that is situated within an artery. Under certain circumstances, balloon expandable stents have been known to come off their stent delivery catheter during insertion into a patient, which phenomenon is referred to as stent embolization. To prevent stent embolization, the stent is typically crimped and then nested onto the balloon of the stent delivery catheter. The process of crimping is accomplished by applying a radial force in an inward direction to the exterior cylindrical surface of the stent. The crimping force squeezes the stent radially inward against the outer surface of the balloon, compressing the balloon under the struts of the stent. The process of nesting which follows crimping has the crimped stent inserted in and restrained by a heated tube while the balloon under the stent is inflated to an elevated pressure with an inert gas or nitrogen. The inflated balloon is then kept at the elevated temperature for a certain period of time which can be called the “nesting time.” For a typical bare metal stent, the crimping force causes the diameter of the balloon under a stent strut to be reduced by approximately 3 mils (1.0 mil is equal to 0.001 inches). The nesting of a bare metal stent is typically accomplished by the application of a temperature of about 95 degrees C., at a pressure into the balloon of about 10 pounds per square inch (psi), for a nesting time period of about 20 seconds. The result of the nesting is to cause the balloon to bulge up into the interstices between the struts of the stent. The bulges then act as a restraining means to prevent longitudinal movement of the stent and thus prevent stent embolization. This phenomenon of the bulging outward of the balloon is clearly seen in FIGS. 2, 3 and 4 of the Fischell et al, U.S. Pat. No. 6,375,660.
The use of crimping and nesting processes is even more important to avoid stent embolization when the stent has certain design features such as wide struts that cannot “dig in” to the balloon material or mostly longitudinal elements that will easily slide along the balloon. An even bigger problem occurs in drug eluting stents where the drug coats the metal stent, e.g., the Cordis Cypher® Stent or the drug elutes from drug reservoirs (holes in the stent) e.g. the Cordis Nevo® Stent. There are several issues here:
1. the drug can be damaged by the elevated pressures used in nesting,
2. the coating itself is relatively soft and delicate thus limiting the pressure that can be used for crimping and
3. even if nesting temperatures that will not damage the drug are used, drug placed in reservoirs will soften from elevated temperatures and may pop out of the holes by the bulging balloon. This can be a significant problem with designs such as the Cordis Nevo® Stent where the wider struts, needed to hold the drug, reduce the ability of the stent to hold onto the balloon as the catheter is advanced into the body.
Fischell, et al in U.S. Pat. Nos. 6,375,660, 6,936,065 and 7,011,673 (“the Fischell patents”) each show elastic bands that are placed over the ends of a stent delivery inflatable balloon that would help prevent stent embolization. The bands of the Fischell patents are shown only for use on fixed wire stent delivery systems and are placed up against the distal and proximal ends of the stent which is impractical unless the bands are placed after the stent is crimped onto the balloon. In addition, the Fischell patents teach only that the bands extend just to the ends of the balloon. This construction can create a bump in diameter of the delivery system thereby reducing the deliverability of the system.